Sumner sits next to 4 phonetic neighbors — Summer, Sommer, and Sunner are the closest matches by sound. Most share Sumner's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"one who summons, court summoner"
"from Devon, deep valley dwellers"
"the Jordan River"
"divine, goddess of the moon"
"rival, eager to equal"
"from Paris; place of the Parisii tribe"
"Rainbow; or kestrel bird"
Similar sound: These names share phonetic patterns with Sumner — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old French origins, giving them the same cultural heritage as Sumner.
Rhythm: Sumner is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.